R - Filter a vector using a function

问题

I have a function similar to this one:

isGoodNumber <- function(X) 
{
if (X==5) return(TRUE) else return(FALSE)
}

I have a vector:
v<-c(1,2,3,4,5,5,5,5)

I want to obtain a new vector that contains the elements of v where isGoodNumber(v) == TRUE

How do I do this ?

Tried v [ isGoodNumber(v) == TRUE ] but it doesn't work :-)

Thanks !!

回答1:

You'll need to Vectorize the function to call it on a vector:

isGoodNumber = Vectorize(isGoodNumber)
v[isGoodNumber(v)]

回答2:

There is a function named "Filter" that will do exactly what you want:

Filter( isGoodNumber, v)
#[1] 5 5 5 5

There would be the option of making a function that was vectorized, either by by the use of the Vectorize function (already illustrated) or writing it with ifelse (also mentioned) and there would be the option of a function that was "Filter"-like

 isGoodNumber3 <- function(X) 
   { X[ ifelse(X==5, TRUE,FALSE)]
   }

 isGoodNumber3(v)
#[1] 5 5 5 5

回答3:

I think here is the easiest method

> v<-c(1,2,3,4,5,5,5,5)
> v[v==5]
[1] 5 5 5 5

回答4:

Use mapply():

> v <- c(1,2,3,4,5,5,5,5)
> newV <- mapply(function(X) { if (X==5) return(TRUE) else return(FALSE) }, v)
> newV
[1] FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE
> v[newV == TRUE]
[1] 5 5 5 5

回答5:

You should get into the habit of writing vectorised functions where possible. For example, if you're writing a function f(x), make sure it works when x is a vector, not just a single number. Don't rely on Vectorize to do this for you, because it will be slow for very large vectors.

A useful technique is to replace if ... then ... else with ifelse. For example:

isGoodNumber <- function(X) 
{
  ifelse(X==5, TRUE, FALSE)
}
v<-c(1,2,3,4,5,5,5,5)
v [ isGoodNumber(v) == TRUE ]

In this particular case you can of course streamline things:

isGoodNumber <- function(X) return(X==5)

or even just

v[v==5]

but the ifelse technique will be useful more generally.

来源：https://stackoverflow.com/questions/9265553/r-filter-a-vector-using-a-function

标签

function

vector